Washing Wedges

Image by Etienne Reyssat/PMMH, ESPCI

The images represent a sequence of top views of a sharp wedge (angle 1°) formed by two almost horizontal glass plates. The apex of the wedge is located along the bottom boundary of the pictures and the width of an image is about 10 cm. The apex of the wedge is initially filled with a small volume of silicone oil (clear liquid).

Image by Etienne Reyssat/PMMH, ESPCI

A surfactant solution (dyed red liquid) is then introduced in the wedge. As it preferentially wets the glass plates, a complex exchange process occurs that leads to the extraction of the oil from the wedge. The oil/water interface destabilizes, oil drops are released and, driven by the confinement gradient, migrate away from the tip of the wedge through the water phase.

Image by Etienne Reyssat/PMMH, ESPCI

The sequence shows different stages of the exchange process of the oil and soap solution. In the present case, the oil viscosity is 5 cSt and the exchange is complete after typically one hour.